As kids grow from pre-pubescence into teens, their rate of physical activity starts declining sharply, US researchers discovered.

It is no wonder that it happens so with so many parents working over hours in order to meet ends, and thus spending less time with their children, encouraging them to be more active or do physical activity together. Also, some teachers do not encourage physical activity skipping over hours in the curricula when they should take children outside the classroom and make them more aware that exercising is vital for their health.

Also we live in world where safety is not guaranteed for our children, with some parents trying to give them alternatives for spending time inside the house thinking the kids would be more safe then if playing outside.

Finally yet importantly, we live in a world where the computer rules, offering teenagers the privilege to abandon outdoor activities for the indoor ones. It is far more comfortable to play just sitting in a chair in front your computer. Or it is far more comfortable to just lay down on your couch and watch TV for hours.

All these being said, it is no wonder that physical activity is something that tends to be rare, as kids become teens. This is the conclusion of a study made by Philip R. Nader, MD, professor emeritus of pediatrics at the University of California San Diego School of Medicine, La Jolla and his colleagues.

For the study, they collected physical activity data on more than 1,000 children who participated in the long-running National Institute of Child Health and Human Development Study of Early Child and Youth Development. The children wore accelerometers, computerized devices on their belt, which measured their movement. The devices gave researchers an accurate view of how kids move and exercise on weekly basis.

The researchers tracked children beginning at age 9 and then again at ages 11, 12 and 15. At age 9 and 11, the kids were doing well, getting two extra hours of moderate to vigorous physical activity above the 60-minute minimum recommended by most doctors.

However, things changed by the time the kids reached 15, with only 31 percent of them meeting the guideline during the week, and just 17 percent on the weekend. Overall, they were moving only 49 minutes on weekdays and 35 minutes on weekends.

The decline began differently for boys and girls. More exactly, girls fell below the guideline by about age 13.1 for weekday activity and boys at age 14.7.

Overall, boys were a little bit more active compared to girls, moving an average of 18 more minutes a day.

“I was surprised by the degree of the drop; it’s a dramatic shift. Younger children appear to be naturally active, but as kids get older, they find fewer opportunities to be active,” Prof. Nader said.

The findings should work as a wake-up call to pediatricians, parents, and policy makers. It is well known that inactivity is linked to greater risks for many health problems, including heart disease, obesity, high blood pressure, and diabetes. About 25 million US children and teens are estimated to be overweight or obese.

Moreover, according to the American Diabetes Association, about 176,500 children and adolescents younger than 20 have diabetes, and 2 million teenagers have blood glucose levels higher than normal, a condition called pre-diabetes.

“Parents have to really take a hard look at what their own activity patters are and really plan things as a family,” Prof. Nader encouraged. “Even taking a walk 15 minutes a day would really start reversing some of these trends. Schools are still cutting out physical education and recess. Parents should get active about that. They shouldn’t allow that happen,” he added.

The study, funded by the National Institute if Child and Human Development, was published in the July 16 issue of The Journal of the American medical Association.